The invention relates generally to radiographic detectors for diagnostic imaging, and more particularly to a multi-layer, direct conversion computed tomography (CT) detector for high flux rate imaging with photon counting and energy discrimination.
Radiographic imaging systems, such as x-ray and computed tomography (CT) have been employed for observing, in real time, interior aspects of an object. Typically, the imaging systems include an x-ray source that is configured to emit x-rays toward an object of interest, such as a patient or a piece of luggage. A detecting device, such as an array of radiation detectors, is positioned on the other side of the object and is configured to detect the x-rays transmitted through the object. As will be appreciated, the intensity of the attenuated beam radiation received at the array of detectors is typically dependent upon the attenuation of the x-rays by the object. Each detector element of the array of detectors is configured to produce a separate electrical signal indicative of the attenuated beam received by the respective detector element. The electrical signals are then transmitted to a data processing system for analysis and image production.
Conventional CT imaging systems utilize detectors that convert radiographic energy into current signals that are integrated over a time period, then measured and ultimately digitized. A drawback of such detectors however is their inability to provide data or feedback as to the number and/or energy of photons detected. As will be appreciated by one skilled in the art, typically, conventional CT detectors have a scintillator component and photodiode component. The scintillator component illuminates upon radiation by radiographic energy. Further, the photodiode detects illumination of the scintillator component and provides an electrical signal as a function of the intensity of illumination. These energy discriminating, direct conversion detectors are capable of not only x-ray counting, but also providing a measurement of the energy level of each x-ray detected. Typically, semiconductor materials have been used in the construction of direct conversion energy discriminating detectors, while other materials may also be employed in the construction of these detectors.
However, a drawback of these direct conversion semiconductor detectors is that these types of detectors cannot count at the x-ray photon flux rates typically encountered with conventional CT systems. Further, the very high x-ray photon flux rate has been known to cause pile-up and polarization that ultimately leads to detector saturation. In other words, these detectors typically saturate at relatively low x-ray flux level thresholds. Above these thresholds, the detector response is not predictable or has degraded dose utilization.
Further, as will be appreciated, detector saturation leads to loss of imaging information and consequently results in artifacts in x-ray projection and CT images. In addition, hysteresis and other non-linear effects occur at flux levels near detector saturation as well as flux levels over detector saturation. As previously noted, direct conversion detectors are also susceptible to a phenomenon called “polarization” where charge trapping inside the material changes the local electric field, alters the detector count and energy response in an unpredictable way, and results in hysteresis where response is altered by previous exposure history. In particular, photon counting, direct conversion detectors, saturate due to the intrinsic charge collection time (i.e., dead time) associated with each x-ray photon event. Saturation occurs due to pulse pile-up when the x-ray photon absorption rate for each pixel is on the order of the inverse of this charge collection time. The charge collection time is approximately proportional to the smaller of either the thickness of the direct conversion layer for a fixed electric field or the anode contact size; therefore, an increase in saturation rate is possible if the direct conversion layer is thinner. However, a sufficient thickness is desirable to stop almost all the x-rays and thus, optimize dose utilization. Incomplete collection of x-rays results in reduced image quality, i.e., a noisy image.
In addition, detectors that measure x-ray photon count rate and energy are subject to a count rate saturation limit. This limit is related to the charge collection time for transport across the detector thickness. Thin detectors allow for rapid charge collection, but they do not have sufficient stopping power to capture the x-rays efficiently when the x-rays are incident along the thin dimension. Therefore, in conventional detectors, a single layer with relatively large thickness (e.g., greater than 1 mm) is used to achieve high efficiency. However, this leads to large charge collection time and associated low flux rate saturation limits. Another disadvantage of thick layer detectors is that charge trapping is more likely during transport through a thick layer. Trapped charge changes the internal electric field and consequently alters the detector gain and spectral response. Transport across a thick layer is also associated with charge sharing between pixels. X-rays that are collected near a boundary between two pixels are shared between these pixels leading to miscounting of the number of incident photons, or incorrect registration of the photon energy. Thick layers are also difficult to create by a deposition technique.
Conventionally, a direct conversion detector is typically made from a single layer. An electric field is applied across the thickness of the layer by applying voltage to contacts on the faces of the layer. The layer is oriented with the x-rays normal to its face. Charge transport occurs across the thickness of the layer. During this charge transport, crosstalk and charge trapping occurs and the incomplete collection of charge causes changes in the detector response. However, if the pixel contact dimension is small relative to the thickness (e.g., less than half the thickness), the charge collection time is less sensitive to the layer thickness and is instead largely a function of the pixel contact size as a result of the “small pixel” effect. However, this small pixel effect does not improve the polarization; charge trapping still is as likely to occur during transport across the thickness of the detector layer. In addition, small pixels are subject to greater charge sharing between pixels.
Previously conceived solutions to enable photon counting at high x-ray flux rates include using sub-mm pixel size to achieve lower count rate per pixel and/or using stacked laminated multiple layer detectors to get lower count rates from each detector layer. However, for photon counting, direct conversion detectors with sub-mm pixel size, Detector Quantum Efficiency (DQE) loss due to charge sharing will be disadvantageously significant. Additionally, if the detector works in the energy discrimination mode to count x-rays from two energy bins for material decomposition, the increased charge sharing due to the sub-mm pixel size causes more spillover counts from the high energy window to the lower energy window, thereby degrading the material decomposition performance. Furthermore, employing the stacked multiple layer detector results in non-uniform x-ray sharing in different detector layers since x-rays attenuate exponentially in the detector and the attenuation coefficient is a strong function of x-ray energy.
In addition, smaller pixels or detector elements have larger perimeter to area ratios disadvantageously resulting in elevated levels of cross talk. The perimeter is a region where charge is shared between two or more pixels. This sharing of charge results in incomplete energy information and/or a miscount of x-ray photons because the readout electronics are not configured to combine simultaneous signals in neighboring pixels. Very high flux rates are possible with thin, photon counting, direct conversion silicon layers with pixel size <0.1 mm, but disadvantageously, these thin layers do not possess sufficient stopping power to stop the x-rays.
Furthermore, motion of electrons and holes contributes to a signal generated by room temperature direct conversion detectors. The relatively low mobility and strong trapping of holes is a cause of degraded detector performance. This degraded detector performance includes non-uniform detector response as function of x-ray absorption depth, polarization and unpredictable and unstable charge collection. Therefore, it is desirable to configure the geometry of the pixelated detector to deemphasize the hole contribution to the detector response by leveraging the small pixel effect. In a pixelated detector, it is desirable to keep the ratio of the pixel contact size to the detector thickness small in order to achieve good small pixel effect. Consequently, in this case the signal from the anode pixel is only proportional to the number of electrons arriving at the anode and independent of the x-ray interaction depth thereby resulting in uniform detector response and enhanced energy resolution. Another advantage of better small pixel effect is the shorter detector dead time due to the fact that the signal current pulse width is determined by the electron drift time across the distance of pixel size instead of detector thickness. However, for the laminated detector configuration with multiple thin layers, the good small pixel effect is no longer achievable using the conventional simple pixelated anode if the pixel contact size is comparable or larger than the detector thickness. Consequently, the detector may experience significant degradation of energy resolution. Additionally, the detector dead time may not be optimized.
It would therefore be desirable to develop a direct conversion, energy discriminating CT detector that does not saturate at the x-ray photon flux rates typically found in conventional CT systems. It would be further desirable to develop a direct conversion, energy discriminating CT detector that advantageously facilitates shorter detector dead time and a substantially uniform and stable detector response, thereby circumventing the limitations of current techniques.